Decay Times of Scintillators

The rate of decay of luminescence following excitation by a high energy particle is an important parameter both in application and interpretation of scintillation phenomena. Two of the present authors developed a more accurate method of measuring scintillation decay, using a pulsed X-ray tube. However, the intensity and time resolution of that apparatus was not adequate for the measurement of all interesting scintillators. A new apparatus has been constructed in which cathode rays are used to excite the scintillator. Cathode-ray pulses as short as 10-10 sec are generated by sweeping a cathode-ray beam across a narrow slit. The emergent electrons in the pulse are accelerated to ~80 key and pass through a thin window to strike the scintillator. The latter may be a crystal, plastic or liquid. The light from the scintillator is detected and amplified by a multiplier phototube, type 1P28. The currents from the anode and last dynode are fed through 120-ohm transmission lines to a traveling-wave oscilloscope. A value of 2.15 × 10-9 sec is obtained for the decay time of scintillations from a solution of 5 g/l of terphenyl in toluene, in agreement with previous measurements. The apparatus is currently being used to study the dependence of the decay time on solute concentration in liquid and plastic scintillators. The decay time is found to increase with decreasing concentration, as predicted by the current theories of energy transfer.